The Superconductive Circuit Board

Understanding PCB Manufacturing: Odd Layer PCB Boards

Our cookbook of PCB recipes includes an assortment of odd layer PCBs.

About 8% of PCB boards we are asked to produce are comprised of an odd number of layers. But unless you have a specific requirement, it is usually best to design a multilayer board with an even number of layers. Reducing an even layered board by one layer may seem like a cost-saving move, but from a PCB perspective it is not. It may actually increase the cost as well as lead-time and leave you with a warped PCB board which may not meet your expectations. Let's look at the issues in detail.

Cost savings:

An odd number of layers means one layer is blank, so no film is needed for that layer. This represents small savings in CAM. If cost savings are the main reason you are entertaining an odd layer design, you may realize more tangible results by reviewing our article ondesign techniques and best practices.

Possible additional costs:

A multilayer board is manufactured by laminating multiple 2 layer boards together into a single stack. They are pressed together at a high temperature for several hours in a special PCB press.

Once the curing process is complete, the PCB boards are allowed to cool before being removed. To make an odd layer PCB, we normally use a standard symmetrical even layer recipe and completely etch away one copper layer.

Different materials contract different amounts during the cool down, so asymmetrical builds will have asymmetrical stresses.

An odd number of layers means one of those 2 layer boards is actually a 1 layer board – traces on one side with all metal etched from the other side. This has a few implications including cost.

Pressing a 1 layer board is like cooking toast over a campfire.

If you toast only one side of the bread, but not the other, you find it will warp. By etching all copper from one side of the PCB board, it tends to make the panel warp when the copper on the other side cools. Warpage causes several problems for production. Limiting warpage in any multilayer PCB board requires care in ensuring a balanced stack-up of base and pre-preg layers, and this is made even more challenging with an odd number of layers. Even when warpage can be controlled in PCB production, you may find it is worse after wave soldering.

One area of misinformation surrounding odd layer count designs is its impact on the copper etching process. There is no risk of over-etching when one side is being completely etched away and the reverse design has finer element but mismatched copper weights is an area of concern in the copper plating process. Having a significantly different weight of copper to plate to each side increases the risk of either under or over plating. It’s a bit like pan frying an egg in a heavy cast iron pan and expecting the top side to cook at the same rate as the pan side.

Alternatives

If there is no design reason to keep to an odd number of layers, then don't.

Use an even number of layers, content in the knowledge that using one less layer doesn't help anyone. If the number of power or signal layers is odd, add another one. Ideally, you want the number of power and signal planes to be even as well.

If you have a 3 layer design, but it doesn't need to be 3 layers, the easier way to achieve copper balance is to just duplicate the inner layer.

If, even after a few cups of coffee, you decide for design reasons you can only live with an odd number of layers, rest assured we can make it work for you.

Additional steps can be taken to try to keep yourmultilayer PCB board within IPC warpage specifications to meet your requirements but you may find it incurs additional cost and time.